The human heart has four valves that control the direction of blood flow through the four chambers of the heart. On the left or systemic side of the heart are the mitral valve, located between the left atrium and the left ventricle, and the aortic valve, located between the left ventricle and the aorta. These two valves direct oxygenated blood coming from the lungs, through the left side of the heart, into the aorta for distribution to the body. On the right or pulmonary side of the heart are the tricuspid valve, located between the right atrium and the right ventricle, and the pulmonary valve, located between the right ventricle and the pulmonary artery. These two valves direct de-oxygenated blood coming from the body, through the right side of the heart, into the pulmonary artery for distribution to the lungs, where it again becomes re-oxygenated to begin the circuit anew. With relaxation and expansion of the ventricles (diastole), the mitral and tricuspid valves open, while the aortic and pulmonary valves close. When the ventricles contract (systole), the mitral and tricuspid valves close and the aortic and pulmonary valves open.
Each of the four valves consists of moveable “leaflets” that are designed to open and close in response to differential pressures on either side of the valve. The mitral and tricuspid valves are referred to as “atrioventricular valves” as they are situated between an atrium and ventricle on each side of the heart. The mitral valve has two leaflets and the tricuspid valve has three. The aortic and pulmonary valves are referred to as “semilunar valves” because of the unique appearance of their leaflets, which are more aptly termed “cusps” and are shaped somewhat like a half-moon. The aortic and pulmonary valves each have three cusps.
Heart valve disease is a widespread condition in which one or more of the valves of the heart fail to function properly. Diseased heart valves may be categorized as either stenotic, wherein the valve does not open sufficiently to allow adequate forward flow of blood through the valve, or incompetent or insufficient, wherein the valve does not close completely, causing excessive backward flow of blood through the valve into the prior chamber when the valve is closed. Both of these conditions increase the workload on the heart and, if left untreated, can lead to debilitating symptoms including congestive heart failure, permanent heart damage and ultimately death. Dysfunction of the left-sided valves—the aortic and mitral valves—is typically more serious since the left ventricle is the primary pumping chamber of the heart.
Dysfunctional valves can either be repaired, with preservation of the patient's own valve, or replaced with some type of mechanical or biologic valve substitute. Since all valve prostheses have some disadvantages (e.g., need for lifelong treatment with blood thinners, risk of clot formation and limited durability), valve repair, when possible, is usually preferable to replacement of the valve. Many dysfunctional valves, however, are diseased beyond the point of repair. In addition, valve repair is usually more technically demanding and only a minority of heart surgeons is capable of performing complex valve repairs. The appropriate treatment depends on the specific valve involved, the specific disease/dysfunction and the experience of the surgeon.
The aortic valve, and less frequently the pulmonary valve, are more prone to stenosis, which typically involves the buildup of calcified material on the valve leaflets, causing them to thicken and impairing their ability to fully open to permit adequate forward blood flow. Most diseased aortic and pulmonic valves are replaced rather than repaired because their function can be easily simulated with a replacement prosthesis and because the typical types of damage to these valves is not easily repairable.
The mitral valve, and less commonly the tricuspid valve, are more commonly affected by leaflet prolapse. While regurgitant mitral valves can be repaired, many are replaced due to the complexities of surgically correcting the underlying redundant valve segments, ruptured chordae, and papillary muscle malposition.
The most common treatment for stenotic valves, particularly aortic valves, is the surgical replacement of the diseased valve. If a heart valve must be replaced, the choice of a particular type of prosthesis (i.e., artificial valve) depends on factors such as the location of the valve, the age and other specifics of the patient, and the surgeon's experiences and preferences. Two major types of prosthetic or replacement heart valves exist: mechanical prostheses and biologic prostheses.
Mechanical prostheses are generally formed entirely of artificial material, such as carbon fiber, titanium, Dacron™ and teflon. There are currently three widely used types of mechanical prostheses: the Starr-Edwards ball-in-cage valve, the Medtronic-Hall tilting disc valve, and the St. Jude bi-leaflet valve. Caged ball valves usually are made with a ball made of a silicone rubber, e.g., SILASTIC™, inside a titanium cage, while bi-leaflet and tilting disk valves are made of various combinations of pyrolytic carbon and titanium. All of these valves are attached to a cloth material sewing ring or mounting cuff so that the valve prosthesis can be sutured to the patient's native tissue to hold the artificial valve in place postoperatively. All of these mechanical valves can be used to replace any of the heart's four valves.
Although mechanical valves have proven to be extremely durable, they all require life-long anticoagulation with blood thinners to prevent clot formation on the valve surfaces. If such blood clots form on the valve, they may preclude the valve from opening or closing correctly or, more importantly, the blood clots may disengage from the valve and embolize to the brain, causing a stroke. The anticoagulant drugs that are necessary to prevent this are expensive and potentially dangerous in that they may cause abnormal bleeding or other side effects. Mechanical valves have the further disadvantage in that the mounting cuffs or sewing rings occupy space, narrowing the effective orifice area of the valve and reducing cardiac output.
The second major type of prosthetic or replacement heart valve is a biologic or tissue valve. These valves include allografts or homografts (usually a valve transplanted from a donor cadaver), autologous grafts (constructed from non-valvular tissue (e.g. pericardium) or from another cardiac valve from the patient himself) and xenografts (animal heart valves typically harvested from cows and pigs). Commercially available biologic tissue valves include the Carpentier-Edwards Porcine Valve, the Hancock Porcine Valve, and the Carpentier-Edwards Pericardial Valve. Recently, there has been an increasing effort to develop synthetic biologically compatible materials to substitute for these natural tissues.
Tissue valves have the advantage of a lower incidence of blood clotting (thrombosis). Hence patients receiving such a valve, unlike those receiving a mechanical valve, do not require prolonged anticoagulation therapy with the potential clinical complications, expense, and patient inconvenience. The major disadvantage of tissue valves is that they lack the long-term durability of mechanical valves. Tissue valves have a significant failure rate, usually appearing at approximately 8 years following implantation, although preliminary results with the new commercial pericardial valves suggest that they may last longer. One cause of these failures is believed to be the chemical treatment of the animal tissue that prevents it from being antigenic to the patient.
Bioprosthetic or tissue valves are provided in stented or unstented forms. A stented valve includes a permanent, rigid frame for supporting the valve, including the commissures, during and after implantation. The frames can take the form of a plastic, wire or other metal framework encased within a flexible fabric covering. Unstented valves do not have built-in commissure supports.
While the stented tissue valves guarantee alignment of the commissures, they cause very high stresses on the commissures when the valve cusps move between open and closed positions. Additionally, the frames or stents can take up valuable space inside the aorta such that there is a narrowing at the site of valve implantation. As with mechanical valves, the frame includes artificial materials which can increase the risk of new infection or perpetuate an existing infection.
In many situations, biologic replacement heart valves are preferred in the unstented form due to the drawbacks mentioned above. Such valves are more resistant to infection when implanted free of any foreign material attachments, such as stents or frames. Despite the known advantages of using biologic prosthetic heart valves without artificial supporting devices such as permanent stents or frames, relatively few surgeons employ this surgical technique due to its high level of difficulty. When unsupported or unstented by a frame or stent, biologic replacement heart valves are flimsy and overly flexible such that the commissures of the heart valve do not support themselves in the proper orientation for implantation. For these reasons, it is very difficult to secure the commissures properly into place. In this regard, the surgeon must generally suture the individual commissures of the heart valve in the exact proper orientation to allow the valve to fully and properly function.
Regardless of the type of valve used, a valve replacement procedure first involves excising the natural valve from the heart. The natural annulus is then sized with a sizing, instrument. After the size has been determined, a valve is then selected for a proper fit. Proper sizing is important as an oversized replacement valve can cause coronary ostial impingement or tearing of the natural annulus. On the other hand, an undersized valve will reduce flow volume and cardiac output. Next, sutures are placed in the natural valve annulus. Usually, a plurality of very long sutures are applied to the annulus, and are carefully laid out to extend through the incision in patient's chest to points outside the incision. Various suture techniques may be used, including simple interrupted, interrupted vertical mattress, interrupted horizontal mattress with or without pledgets, or continuous, depending on the anatomical structure of the valve being replaced, the type of replacement valve being used, and the particular patient's anatomy. Regardless of the specific suturing technique employed, suture placement within the native valve annulus is crucial to the outcome of the valve replacement procedure, requiring accurate and flawless suturing. After placement in the natural valve annulus, working outside of the chest, the same sutures are placed through the valve's mounting cuff or sewing ring, which is provided fixed to the valve itself. The individual sutures are specifically placed on the valve to provide the proper orientation of the valve with respect to the valve annulus. The valve and sewing ring are then “parachuted” or slid down the sutures and seated within the native valve annulus with the proper valve orientation maintained. The sutures anchoring the cuff of the prosthesis to the host tissue are then tied off and the excess suture length trimmed.
While suturing of replacement valves has long been the accepted technique in implanting prosthetic valves, this technique is replete with shortcomings. Suturing of a valve is a very complex procedure, requiring the utmost care and accuracy. As such, improperly suturing a replacement valve is not inconsequential. Correcting inadequately placed valves may require complete removal of the valve (i.e., by cutting the sutures holding the valve) and reseating the valve as described above. Repetition of the suturing process causes excessive perforation of the native valve annulus subjecting it to risk of tearing and may effect the functioning of the replacement valve once permanently placed. This risk also presents itself in subsequent surgeries performed to replace a prosthetic valve suffering from excessive wear or mechanical failure. The sutures holding the prosthetic device in place must be removed and a new device inserted and resutured to the surrounding tissue. After a number of replacements, the tissue surrounding the valve becomes perforated and scarred making attachment of each new replacement valve progressively more difficult for the surgeon and riskier for the patient.
In addition to the complexity of valve suturing, conventional heart valve replacement surgery can be very invasive, involving access to the patient's heart through a large incision in the chest, such as a median stemotomy or a thoracotomy. Since conventional valve replacement procedures involve work inside the heart chambers, a heart lung machine is required. During the operation, while the patient is “on the pump,” the heart is isolated from the rest of the body by clamping the aorta and stopped (cardioplegic arrest) through the use of a high potassium solution. Although most patients tolerate limited periods of cardiopulmonary bypass and cardiac arrest, these maneuvers are known to adversely affect all organ systems. The most common complications of cardiopulmonary bypass and cardiac arrest are stroke, myocardial “stunning” or damage, respiratory failure, kidney failure, bleeding and generalized inflammation. If severe, these complications can lead to permanent disability or death. The risk of these complications is directly related to the amount of time the patient is on the heart-lung machine (“pump time”) and the amount of time the heart is stopped (“cross-clamp time”).
The complex suturing of the prosthetic valve within the valve annulus and the subsequent knot tying involved in valve replacement procedures, as discussed above, is very time consuming, requiring a significant amount of pump time. Because the success of valve replacement can only be determined when the heart is beating, the heart must be closed up and the patient taken off the heart lung machine before verification can be made. If the results are determined to be inadequate, the patient must then be put back on cardiopulmonary bypass and the heart stopped once again.
Recently, a great amount of research has been done to reduce the trauma and risk associated with conventional open-heart valve replacement surgery. A variety of minimally invasive valve repair procedures have been developed whereby the procedure is performed through small incisions with or without videoscopic assistance and, more recently, with robotic assistance. However, the time involved in these minimally invasive procedures is often greater than with conventional valve replacement procedures as the suturing process must now be performed with limited access to the valve and, thus, limited dexterity even in the hands of experienced surgeons.
Other technologies are being developed in the area of cardiac valve replacement with the hope of overcoming the disadvantages of suturing by simplifying the valve attachment procedure and reducing the time necessary to complete such procedure. These proposed technologies include stapling and fastening devices that deploy one or more staples or fasteners at the valve attachment site in a single action. Examples of such technologies are disclosed in U.S. Pat. Nos. 5,370,685, 5,716,370, 6,042,607, 6,059,827, 6,197,054, and 6,402,780. Although stapling and fastening may save time, great precision and accuracy are required to ensure proper placement and alignment of the replacement valve prior to placement of the staples/fasteners. An improperly placed staple or fastener can be very difficult to remove at the risk of tearing or damaging the tissue at the valve site. Another sutureless valve replacement technology which has been disclosed but remains to be clinically proven is that of employing thermal energy, such as radio frequency energy, to shrink the natural valve annulus around a prosthetic valve placed within it. An example of this technology is disclosed in U.S. Pat. No. 6,355,030.
Thus, it is desirable to provide a prosthetic cardiac valve system, the implant of which requires a minimum of amount suturing and preferably no suturing in order to decrease the amount of time the patient's heart would need to be stopped and bypassed with a heart-lung machine. It would be additionally advantageous if such cardiac valve could be removed or its position adjusted once implanted, either at the time of the original implant procedure or in a subsequent operation. It would be additionally desirable if such prosthetic cardiac valve could be implanted without the need for cardiopulmonary bypass and cardioplegic arrest. Still yet, a further advantage would be to provide a prosthetic valve that could be implanted by means of percutaneous or endovascular approaches.